The present invention relates to the field of liquid filtration and, more particularly, to pressure leaf filters. These filters typically comprise a pressure vessel containing a plurality of filter leaves. An unfiltered liquid is introduced into the vessel under pressure where it is forced through the filter leaves. The filter leaves have internal drain connections which permit the filtered liquid to pass to the outside of the pressure vessel. Such filters are commonly called cake filters due to the fact that the removed solids form a cake on the surface of the filter leaves. The cake, in fact, acts as a filter medium. The filter leaves function mainly to retain the cake and to provide sufficient cross-sectional drainage area to enable the filtered liquid to be evacuated as quickly as possible.
In order to function satisfactorily, a filter leaf must have a surface screen which remains flat and tight during operation. For this reason, it is customary to support filter leaf screens on internal drainage members having the necessary structural integrity. In recent years the structural requirements for such drainage members have become increasingly severe, as filter sizes have increased to cope with ever increasing flow requirements and ever decreasing limits on the amount of allowable solids in the filtered liquids. As noted above, filter leaf drainage members must provide both flow and rigidity. They must also be adapted for attachment to other structural members and have the same shape as the surface screens.
Popular drainage members used today are 4.times.4.times.0.080 woven wire screen and tubular slit, which is a piece of sheet metal pressed or formed to provide flow passages. However, the wire screen does not provide sufficient liquid flow for large size leaves, and tubular slit is a weak material in large sheet sizes. The tubular slit tends to buckle and deflect very easily under applied loads and requires special manufacturing assembly procedures to keep it sufficiently flat for properly supporting the surface screens and allowing attachment to a peripheral frame. Such special manufacturing necessarily are quite costly.
It is therefore seen that there is a need for a filter leaf having an improved drainage member which may be produced at low cost and yet provide the surface flatness, structural rigidity and flow rates required in modern filter applications.